The invention relates to a controller for adjusting the operational combustion mixture fed to an internal combustion engine. The controller includes an oxygen sensor and a threshold switch actuated thereby as well as a timing element which generates alternating signals during normal operation and thereby controls a switch to move from a first to a second switching position.
The mixture control to which this invention relates is superimposed on the basic mixture preparation which takes place by means of a carburetor or another system, for example an electronically controlled induction tube injection system. It is the purpose of the present apparatus to use the information provided by the oxygen sensor to maintain the fuel-air mixture fed to the engine at a concentration slightly sub-stoichiometric, i.e., somewhat leaner than the stoichiometric value. Such a mixture results in a particularly favorable exhaust gas composition, especially regarding toxic components. In addition, a fuel-air mixture of this composition permits a catalytic reactor to process the exhaust gases of the engine in a favorable manner.
On the other hand, a control system of this type functions satisfactorily only if the signal generated by the oxygen sensor provides a useable signal. A useable signal is obtained when the oxygen sensor is heated to a temperature exceeding 400.degree. C. If the temperature is substantially less, the resistance of the solid electrolyte, which produces the sensor signal based on the principle of ion conduction and whenever there is present a partial pressure of oxygen, becomes so large that the sensor is no longer able to generate a signal useable for control. This condition obtains, for example, during the warm-up phase of the engine operation. However, in overrunning operation, i.e., during engine braking, or even during extended idling, the exhaust gas temperature may decline so that the minimum operational temperature of the oxygen sensor is no longer attained.
It is known in the control system described above to trigger a warning device or some other mechanism for changing the operation of the engine when the alternating output signals of the oxygen sensor are found to be absent or are otherwise disturbed. It is also known to employ a thermal transducer which is located directly adjacent to the oxygen sensor and which controls the operation of a heating coil for heating the sensor. Such supplementary heating warms up the sensor to its minimum temperature. This system has the disadvantage that the temperature sensor must operate at temperatures far in excess of 400.degree. C. and is relatively expensive. Furthermore, the sensor must operate in the difficult environment found in the exhaust gas of an internal combustion engine and quickly deteriorates.